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Influence of Tamoxifen on Plasma Levels of Insulin-like Growth Factor I and Insulin-like Growth Factor Binding Protein I in Breast Cancer Patients¹

Departments of Oncology [P. E. L.], Internal Medicine [E. A. L.], and Biochemical Endocrinology [A. A.], Haukeland University Hospital, N-5021 Bergen, Norway and Department of Endocrinology, Karolinska Sjukhuset, S-104 01 Stockholm, Sweden [K. H.]

Plasma levels of insulin-like growth factor I (IGF-I) and insulin-like growth factor binding protein I (IGFBP-I) were measured in fasting blood samples obtained from 16 postmenopausal breast cancer patients before and during tamoxifen treatment for 1 to 6 months. Tamoxifen suppressed total plasma IGF-I by a mean of 28.5% (P < 0.001) but elevated plasma IGFBP-I by a mean value of 78% (P < 0.001). Changes in plasma levels of growth hormone, insulin, or insulin C-peptide were not observed. These findings suggest that tamoxifen exerts its influence on plasma IGF-I and IGFBP-I by mechanisms other than those known to regulate the plasma levels of these peptides, primarily growth hormone and insulin, respectively. A dual effect suppressing plasma IGF-I and elevating plasma IGFBP-I suggests that tamoxifen may have a significant influence on endocrine and possibly paracrine delivery of IGF-I to breast cancer cells in vivo.

¹ Supported by grants from the Norwegian Cancer Society and the Swedish Medical Research Council.

² To whom requests for reprints should be addressed.

Received 2/28/92. Accepted 6/19/92.

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